Correlation of radiographic and telemetric data from massive implant fixations

One proximal femoral replacement (PFR) and two distal femoral replacements (DFR) used in tumoural limb salvage were instrumented to measure axial force in the femoral shaft and at the tip of the intramedullary stem in vivo. The prostheses were implanted in suitable subjects and regular recording sessions held over 2 years for each subject. The ratio of the tip force to the shaft force increased with time in all 3 subjects, albeit at different rates, indicating that a greater proportion of the load was being transmitted along the prosthesis stem. The aim of this study was to correlate these changes with bone remodelling observed radiographically over the same period. Radiographs were analysed using a computer-assisted method to quantify bone width, radiolucent line width and relative bone density. The analysis showed the development of radiolucent lines at the bone-cement interface and a decrease in the lateral to medial ratio of bone density near the transection site. For the PFR, the mean width of the radiolucent area alongside the proximal fifth of the stem on the lateral side increased from 0 to 0.67 mm over the 2 years. The tip to shaft ratio correlated significantly with radiolucency on the lateral side of the stem. Both the DFRs became loose within 2 years but the PFR has remained in situ for 13 years to date. The correlations found thus far suggest that it may be possible to establish relationships between load transfer (and therefore loosening in the fixation) and radiographic data, with possible application to advance warning of clinical loosening in stemmed implants.